Developed toner images in electrostatographic processes can be transferred and fused to another substrate such as paper. Transfer of the toner image can be accomplished by electrostatic methods, pressure contact, or other means. Once transferred, the toner image can be fused or fixed to the paper. The fusing step commonly consists of passing the paper on which toner powder is distributed in an imagewise pattern, through the nip of a pair of rolls, at least one of which is heated. The heated roller is often referred to as a fusing roller.
Toner fusing rolls have a cylindrical core, which may contain a heat source in its interior, and a resilient covering layer formed directly or indirectly on the surface of the core. Roller coverings are commonly fluorocarbon polymers or silicone polymers, such as poly(dimethylsiloxane) polymers, of low surface energy that minimize adherence of toner to the roller.
Frequently release oils composed of, for example, poly(dimethylsiloxanes), are also applied to the roller surface to prevent adherence of toner to the roller. Such release oils may interact with the roller surface upon repeated use and in time cause swelling, softening and degradation of the roller. Silicone rubber covering layers, which are insufficiently resistant to release oils and cleaning solvents, are also susceptible to delamination of the roller cover after repeated heating and cooling cycles. The degradation due to the combination of oil interaction and repeated heating and cooling eventually render the roller useless. It is desirable to increase this lifetime as long as possible.
Another persistent problem in this operation is that when the toner is heated during contact with the fusing roller, it may adhere not only to the paper but also to the fusing member. Any toner remaining adhered to the member can cause a false offset image to appear on the next sheet and can also degrade the fusing roller. Any toner or dirt stuck to the roller should be easily removable.
In the past, fusing rollers often had to be cleaned several times before their useful life ran out. This meant that printing time was wasted while somebody physically opened the machine and wiped down the fusing roller. In the case of the high-speed reel paper printers we tested the roller in, the fusing rollers had to be cleaned once every 25,000 copies. This translates to about every 2 days or so, the printers had to be opened and the fusing rollers cleaned. This cleaning was necessary in spite of the fact that the machines had an internal cleaning web which wiped the surface down and applied oil. In addition, the fusing rollers were changed altogether every 250,000 copies, or every 2 weeks or so.